A mass spectrometer is an analytical instrument that separates and detects ions according to their mass-to-charge ratio. Mass spectrometers can be differentiated based on whether trapping or storage of ions is required to enable mass separation and analysis. Non-trapping mass spectrometers do not trap or store ions, and ion densities do not accumulate or build up inside the device prior to mass separation and analysis. Common examples in this class are quadrupole mass filters and magnetic sector mass spectrometers in which a high power dynamic electric field or a high power magnetic field, respectively, are used to selectively stabilize the trajectories of ion beams of a single mass-to-charge (m/q) ratio. Trapping spectrometers can be subdivided into two subcategories: Dynamic Traps, such as for example the quadrupole ion traps (QIT) of Paul's design, and static traps, such as the more recently developed electrostatic confinement traps. Electrostatic traps that are presently available, and used for mass spectrometry, generally rely on harmonic potential trapping wells to trap ions into ion-energy-independent oscillations within the trap, with oscillation periods related only to the mass-to-charge ratio of the ions. Mass analysis in some modern electrostatic traps has been performed through the use of remote, inductive pick up and sensing electronics and Fast Fourier Transform (FFT) spectral deconvolution in Fourier transform mass spectrometry (FTMS). Alternatively, ions have been extracted, at any instant, by the rapid switching off of the high voltage trapping potentials. All ions then escape, and their mass-to-charge ratios are determined through time of flight (TOF) analysis. Some recent developments have combined the trapping of ions with both dynamic (pseudo) and electrostatic potential fields within cylindrical trap designs. In these designs, Quadrupole radial confinement fields are used to constrain ion trajectories in a radial direction while electrostatic potentials wells are used to confine ions in the axial direction into substantially harmonic oscillatory motions. Resonant excitation of the ion motion in the axial direction is then used to effect mass-selective ion ejection.
The PCT/US2007/023834 application by Ermakov et al. discloses an electrostatic ion trap that confines ions of different mass-to-charge ratios and kinetic energies within an anharmonic potential well. The ion trap is also provided with a small amplitude AC drive that excites confined ions. The amplitudes of oscillation of the confined ions are increased as their energies increase, due to an autoresonance between the AC drive frequency and the mass-dependent natural oscillation frequencies of the ions, until the oscillation amplitudes of the ions exceed the physical dimensions of the trap and the mass-selected ions are detected, or the ions fragment or undergo any other physical or chemical transformation.
Autoresonance is a persisting phase-locking phenomenon that occurs when the driving frequency of an excited nonlinear oscillator slowly varies with time. With phase-lock, the frequency of the oscillator will lock to and follow the drive frequency. That is, the nonlinear oscillator will automatically resonate with the drive frequency. In this regime, the resonant excitation is continuous and unaffected by the oscillator's nonlinearity. Autoresonance is observed in nonlinear oscillators driven by relatively small external forces, almost periodic with time. If the driving frequency is slowly varying with time (in the right direction determined by the nonlinearity sign), the oscillator can remain phase-locked but on average increases its amplitude with time. This leads to a continuous resonant excitation process without the need for feedback. The long time phase-lock with the perturbation leads to a strong increase in the response amplitude even under a small driving amplitude. The driving amplitude is related to the frequency sweep rate, with an autoresonance threshold proportional to the sweep rate raised to the ¾ power.
An electrostatic ion trap disclosed by Ermakov et al. included two cylindrically symmetric cup electrodes that were held at ground (0 VDC) potential, and a planar aperture trap electrode, held at a negative DC potential (typically −1000 VDC), located midway between the cup electrodes. This ion trap was entirely cylindrically symmetric, with on-axis ionization of gas molecules and atoms by impact with electrons transmitted from a hot filament into the trap, AC excitation of the ions by application of a small amplitude RF potential to one of the cup electrodes, and detection of the mass-selectively ejected ions by an on-axis electron multiplier device. This design produced good quality spectra of high vacuum environments (pressures lower than 10−6 Torr), but produced noisy spectra with substantial baseline offsets and loss of spectral resolution in higher pressure (10−4-10−5 Torr) environments.